Tetraphenylthiophene-Functionalized Poly(N-isopropylacrylamide): Probing LCST with Aggregation-Induced Emission

The preparation of hexaphenylsilole-based conjugated microporous polymer for fluorescence sensing o-nitrophenol

A first hexaphenylsilole-based conjugated microporous polymer, termed THPS, was prepared by a Friedel-Crafts arylation reaction of 1,1,2,3,4,5-hexabenylthirole with 2,4,6-trichloro-1,3,5-triazine catalyzed by anhydrous aluminium trichloride (AlCl3) in 1,2-dichloroethane. Its Brunauer-Emmett-Teller and Langmuir surface area reached up to 1579 and 2081 m2 g-1, and total pore volume and micropore pore volume were severally 0.8325 and 0.7322 cm3 g-1. THPS could fluorescence-sense o-nitrophenol with high sensitivity and selectivity. Its fluorescence quenching constant (KSV) and limits of detection (LODs) were severally 8.32 × 103 L mol-1 and 3.61 × 10-10 mol L-1. The fluorescence quenching of THPS by o-nitrophenol was originated from the combined action of both photo-induced electron transfer mechanism and resonance energy transfer mechanism.

Diels-Alder Polymer Networks with Temperature-Reversible Cross-Linking-Induced Emission

A novel synthetic strategy gives reversible cross-linked polymeric materials with tunable fluorescence properties. Dimaleimide-substituted tetraphenylethene (TPE-2MI), which is non-emissive owing to the photo-induced electron transfer (PET) between maleimide (MI) and tetraphenylethene (TPE) groups, was used to cross-link random copolymers of methyl (MM), decyl (DM) or lauryl (LM) methacrylate with furfuryl methacrylate (FM). The mixture of copolymer and TPE-2MI in DMF showed reversible fluorescence with "on/off" behavior depending on the Diels-Alder (DA)/retro-DA process, which is easily adjusted by temperature. At high temperatures, the retro-DA reaction is dominant, and the fluorescence is quenched by the photo-induced electron transfer (PET) mechanism. In contrast, at low temperatures, the emission recovers as the DA reaction takes over. A transparent PMFM/TPE-2MI polymer film was prepared which shows an accurate response to the external temperature and exhibited tunable fluorescent "turn on/off" behavior. These results suggest the possible application in areas including information security and transmission. An example of invisible/visible writing is given.

Functional Polymer Systems with Aggregation-Induced Emission and Stimuli Responses

Functional polymer systems with stimuli responses have attracted great attention over the years due to their diverse range of applications. Such polymers are capable of altering their chemical and/or physical properties, such as chemical structures, chain conformation, solubility, shape, morphologies, and optical properties, in response to single or multiple stimuli. Among various stimuli-responsive polymers, those with aggregation-induced emission (AIE) properties possess the advantages of high sensitivity, fast response, large contrast, excellent photostability, and low background noise. The changes in fluorescence signal can be conveniently detected and monitored using portable instruments. The integration of AIE and stimuli responses into one polymer system provides a feasible and effective strategy for the development of smart polymers with high sensitivity to environmental variations. Here, we review the recent advances in the design, preparation, performance, and applications of functional synthetic polymer systems with AIE and stimuli responses. Various AIE-based polymer systems with responsiveness toward single physical or chemical stimuli as well as multiple stimuli are summarized with specific examples. The current challenges and perspectives on the future development of this research area will also be discussed at the end of this review.

A Boron Difluoride Hydrazone (BODIHY) Polymer Exhibits Aggregation-Induced Emission

Polymers that exhibit aggregation-induced emission (AIE) find use, for example, as cell-imaging agents and as fluorometric sensors due to their unique optical properties. However, the structural diversity of AIE-active polymers has not necessarily advanced at the same rate as their applications. In this work, ring-opening metathesis polymerization is used to synthesize the first example of a polymer (M_n  = 61,600 g mol^-1 , Đ = 1.32) containing boron difluoride hydrazone (BODIHY) heterocycles in its repeating unit. The BODIHY monomer and polymer described absorb and emit in the visible region in solution (λ_abs  = 428 and 429 nm, λ_em  = 528 and 526 nm) and as thin films (λ_abs  = 443 and 440 nm, λ_em  = 535 and 534 nm). Monomer (Φ_Film  = 10%) and polymer (Φ_Film  = 6%) exhibit enhanced emission as thin films compared to solution (Φ_Soln  ≤ 1%) as well as AIE upon the addition of water to DMF solutions as a result of restriction of intramolecular motion. Enhancement factors for the monomer and polymer are determined to be 58 and 15, respectively. The title BODIHY polymer exhibited an earlier onset of AIE and enhanced sensitivity to solution viscosity when compared to the parent monomer.

One-Step Multicomponent Polymerizations for the Synthesis of Multifunctional AIE Polymers

As a new class of functional luminescent materials, polymers with aggregation-induced emission (AIE) feature attract much attention because of their advantages of efficient solid-state fluorescence, excellent processability, structural diversity, and multifunctionalities. Among all polymerization methods toward AIE polymers, multicomponent polymerizations (MCPs) exhibit the merits of simple operation, good atom economy, high polymerization efficiency, broad functional-group tolerance, etc. In this feature article, the recent progress on the development of one-step MCPs for the synthesis of AIE polymers is highlighted. The representative functionalities of the resulting AIE polymers are illustrated. Perspectives on the challenges and future development directions of this field are also discussed.

Preserving High-Efficiency Luminescence Characteristics of an Aggregation-Induced Emission-Active Fluorophore in Thermostable Amorphous Polymers

Luminophores usually suffer from luminescent quenching when introduced into a polymer backbone or side chain, which leads to the inefficient luminescence or even no luminescence of the polymer. In this work, alicyclic imide rings were found to be capable of balancing the donor-acceptor properties between the rigid spacer and the aggregation-induced emission-active fluorophore in light-emitting polymers. Along with the nonplanar and rigid emitter, the suppressed intramolecular charge-transfer effect and interchain disturbance can efficiently preserve the luminescence characteristics of the active center, resulting in high solid-state photoluminescence quantum yields of up to 89%. The amorphous polyimides exhibit excellent thermal properties, such as high glass transition temperature (T_g) values (398 °C) and high thermal decomposition temperature (T_d) values (538 °C). As far as we know, these luminescent polymer materials are of excellent heat resistance with the highest luminescence efficiency reported. The results have significant impact for the precise prediction of the optical properties of light-emitting polymers by appropriate monomer design, providing controllable ways for synthesizing high thermal stability polymeric materials with efficient fluorescence properties.

The influences of the structure of thiophene-based conjugated microporous polymers on the fluorescence sensing properties

Three thiophene-based conjugated microporous polymers (CMPs: TTPTh, DBTh, and TBTh) were prepared by Sonogashira–Hagihara cross-coupling polymerization, and their structures were characterized by FTIR, ss ¹³C NMR, and elemental analyses.

Temperature and pH-Dual Responsive AIE-Active Core Crosslinked Polyethylene-Poly(methacrylic acid) Multimiktoarm Star Copolymers

A series of aggregation-induced emission (AIE) active core crosslinked miktoarm star copolymers, having multi polyethylene (PE) and poly(methacrylic acid) (PMAA) arms, were synthesized and their thermal/pH responsive properties were studied. The procedure involves (a) the synthesis of PE-Br by polyhomologation of dimethylsulfoxonium methylide with triethylborane as initiator, followed by oxidation-hydrolysis/esterification reactions and of poly(tert-butyl methacrylate) (PtBMA-Br) by atom transfer radical polymerization (ATRP) of tert-butyl methacrylate, (b) the synthesis of (PE)_n-(PtBMA)_m-P(TPE-2St) by ATRP of a double styrene-functionalized tetraphenylethene (TPE-2St) with PE-Br and PtBMA-Br macroinitiators, and (c) the hydrolysis of (PE)_n-(PtBMA)_m-P(TPE-2St) to afford the amphiphilic miktoarm star copolymers (PE)_n-(PMMA)_m-P(TPE-2St). Due to their spherical core-shell structure (temperature-responsive) and the presence of hydrophilic PMAA (pH-responsive) and TPE-2St (AIE), these miktoarm star copolymers are AIE materials with temperature/pH-dual responsivity. In addition, thanks to the coexistence of hydrophilic and hydrophobic arms, these materials promote stable water-in-oil emulsions.

A covalent triazine-based framework from tetraphenylthiophene and 2,4,6-trichloro-1,3,5-triazine motifs for sensing o-nitrophenol and effective I2 uptake

A covalent triazine-based framework with a tetraphenylthiophene (TTPT) backbone was prepared by the AlCl₃ catalyzed Friedel–Crafts reaction of commercially available material 2,4,6-trichloro-1,3,5-triazine with tetraphenylthiophene in dichloromethane.

Polyacrylic esters with a “one-is-enough” effect and investigation of their AIEE behaviours and cyanide detection in aqueous solution

Four end-functionalized polymers bearing chains with different hydrophilicities were successfully synthesized using a red-light emitting initiator via atom transfer radical polymerization (ATRP).

Polyurethanes with aggregation-enhanced emission characteristics: preparation and properties

An amino-terminated poly(propylene glycol)-modified tetraaryl-buta-1,3-diene derivative (TABDAA) was introduced to synthesize polyurethanes with different ratios of soft/hard segments. A mixture of TABDAA and poly(tetrahydrofuran) 1000 as the soft segments was reacted with 4,4-diphenylmethane diisocyanate and 1,4-butanediol as the hard segments in molar ratios of 1 : 2 : 1, 2 : 3 : 1, and 3 : 4 : 1 to give the desired polyurethanes named TMPU-211, TMPU-321 and TMPU-431, respectively. The three polyurethanes exhibited different aggregation-enhanced emission (AEE) behaviors because of their different soft/hard segment ratios. The polyurethanes with a higher soft segment content tended to form bigger particles in a DMF/water mixture solution, thus causing a sharper increase in their fluorescence intensity. In addition, the polyurethane films exhibited different fluorescence intensities after different heat treatments. After a quenching treatment of the soft segments in the polyurethane films, the fluorescence intensity dropped greatly. When these quenched polyurethane films were thermally annealed at 60 °C for 24 hours, their fluorescence intensity exceeded the initial intensity of the as-prepared films. Differential scanning calorimetry results showed that the polyurethane films in the quenched condition did not present the endothermal melting peak of the soft segments, and the melting peaks appeared again after thermal annealing. AFM experiments showed that an ordered arrangement was achieved after the heat treatment of these AEE polyurethane films. These results demonstrated that the polymer structure had a significant effect on the AEE properties of the polyurethane films, and more importantly, it is of great significance in improving the fluorescence emission of the AEE polymers and also for their potential application in fluorescent probes, stimuli-responsive materials, PLED devices and so on.

Ionic complex of a rhodamine dye with aggregation-induced emission properties

An AIE-active rhodamine based luminogen was prepared via a complexation reaction between non-emissive rhodamine hydrazide (RdH) and bulky camphorsulfonic acid (CSA). Besides acting to open the spirolactam ring of RdH, CSA also imposes a rotational restriction on the resultant ionic complex, RdH(CSA)_x. Without CSA, the analogous complex RdH(HCl)₃ is a luminogen with aggregation-caused quenching (ACQ) properties. The ionic bonds of RdH(CSA)₃ are sensitive to several external stimuli and therefore it is a luminescent sensor for metal ions, organic amines and the blood protein bovine serum albumin (BSA). Besides being a sensor for BSA, the ionic RdH(CSA)₃ is also a denaturant capable of uncoiling the peptide chain of BSA.

Carbon dioxide affects the phase transition of poly(N-isopropylacrylamide)

The effects of atmospheres of CO₂ and N₂ on the LCST of PNIPAAm in aqueous solution using high-pressure differential scanning calorimetry (HP-DSC).

Aggregation-induced emission polymer nanoparticles with pH-responsive fluorescence

A convenient and efficient method was developed to prepare stable polymer nanoparticles with varied morphology and pH-responsive AIE properties, based on the amphiphilic block copolymer self-assembly, crosslinking, and post-functionalization techniques.

Influence of the secondary structure on the AIE-related emission behavior of an amphiphilic polypeptide containing a hydrophobic fluorescent terminal and hydrophilic pendant groups

AIE-related emission of polypeptide containing an AIE-active terminal is correlated with secondary structures (α-helix, β-sheet and random coil) of the peptide chains.

Multiple-responsive ionic complex luminogen of quinine and camphorsulfonic acid with aggregation-induced emission

Bulky camphorsulfonic acid (CSA) was used to complex with quinine (Qu) to impose restricted intramolecular rotation (RIR) required for aggregation-induced emission (AIE) properties.

Polymeric AIE-based nanoprobes for biomedical applications: recent advances and perspectives

The development of polymeric luminescent nanomaterials for biomedical applications has recently attracted a large amount of attention due to the remarkable advantages of these materials compared with small organic dyes and fluorescent inorganic nanomaterials. Among these polymeric luminescent nanomaterials, polymeric luminescent nanomaterials based on dyes with aggregation-induced emission (AIE) properties should be of great research interest due to their unique AIE properties, the designability of polymers and their multifunctional potential. In this review, the recent advances in the design and biomedical applications of polymeric luminescent nanomaterials based on AIE dyes is summarized. Various design strategies for incorporation of these AIE dyes into polymeric systems are included. The potential biomedical applications such as biological imaging, and use in biological sensors and theranostic systems of these polymeric AIE-based nanomaterials have also been highlighted. We trust this review will attract significant interest from scientists from different research fields in chemistry, materials, biology and interdisciplinary areas.

Crystallization-promoted emission enhancement of poly(l-lactide) containing a fluorescent salicylideneazine center with aggregation-enhanced emission properties

AEE-active 1,2-bis(2,4-dihydroxybenzylidene)hydrazine initiates polymerization of lactide monomer, resulting in polylactides containing an AEE-active center.

Rigid Jeffamine-included polyrotaxane as hydrogen-bond template for salicylideneazine with aggregation-enhanced emission

A Jeffamine-included polyrotaxane (JCD) was used as rigid template to impose effective rotational restriction on the AIE-active luminogen of 1,2-bis(2,4-dihydroxybenzylidene)hydrazine (CN₄OH).

Tuning the fluorescence of aggregates for end-functionalized polymers through varying polymer chains with different polarities

Two pyrazoline initiators and their four end-functionalized polymers have been prepared and their tunable emission behaviors have been investigated.

Luminescent polymers and blends with hydrogen bond interactions

Intermolecular hydrogen bonds are effective forces in hampering molecular rotation and in enhancing emission of luminogenic polymers and blends with aggregation-induced emission property.

Aggregation-Induced Emission Rotors: Rational Design and Tunable Stimuli Response

A novel molecular design strategy is provided to rationally tune the stimuli response of luminescent materials with aggregation-induced emission (AIE) characteristics. A series of new AIE-active molecules (AIE rotors) are prepared by covalently linking different numbers of tetraphenylethene moieties together. Upon gradually increasing the number of rotatable phenyl rings, the sensitivity of the response of the AIE rotors to viscosity and temperature is significantly enhanced. Although the molecular size is further enlarged, the performance is only slightly improved due to slightly increased effective rotors, but with largely increased rotational barriers. Such molecular engineering and experimental results offer more in-depth insight into the AIE mechanism, namely, restriction of intramolecular rotations. Notably, through this rational design, the AIE rotor with the largest molecular size turns out to be the most viscosensitive luminogen with a viscosity factor of up to 0.98.

Protein quantitation by complexation of fluorescent tetraphenylthiophene cation to anion-terminated poly(N-isopropylacrylamide): aggregation-enhanced emission and electrostatic interaction

A fluorescent biological sensor utilizing aggregation-enhanced emission (AEE) property was developed in our laboratory. First, the AEE-active fluorescent tetraphenylthiophene (TP) unit was synthetically connected to poly(N-isopropylacrylamide) by covalent and ionic bonds, resulting in the respective c- and i-TP-PNIPAM for the detection and quantification of the bovine serum albumin (BSA) model protein. When bind to BSA, the ionic i-TP-PNIPAM shows much better fluorescence (FL) sensitivity compared to c-PNIPAM. The fluorescence (FL) intensity of i-TP-PNIAPM displays a good linear dependence on concentration of BSA (0-1 mg/mL), indicating quantitative fluorimetric protein detection can be achieved. Further addition of anionic surfactant of sodium dodecylsulfate (SDS) considerably raised the FL intensity of the complex solution. All the FL response was discussed in term of conformational freedom of the TP unit under different environmental constraints.